We can identify a liquid by sensing its properties. For example, tap water flows relatively freely, has little or no flavor, and is colorless. Cooking oil flows more slowly, may have a golden or yellowish tint, and (depending on the kind of oil) may have a certain characteristic flavor and/or odor. Molasses has a still different characteristic color, odor, and taste, and is more resistant to flow.
In addition to color, odor, flavor and flow resistance, people interested in the properties of liquids are often concerned about the density of the liquid. For example, we all know that oak is more dense than balsa wood. Similarly, different liquids can have different densities. Knowing the density of a liquid is often very important.
As one example, salt water is more dense than fresh water because the dissolved salt increases the water's density. It is possible to determine the amount of salt dissolved in a solution by measuring the density of the liquid solution. It is usually most convenient to determine the density of a liquid relative to the density of pure water. This relative density is called "specific gravity." An instrument called a "hydrometer" is used to measure specific gravity.
Some non-technical people may never have heard of a hydrometer, and might think that it is a highly technical instrument found only in the laboratory. But hydrometers are actually quite common. For example, auto mechanics use a kind of hydrometer to check whether car batteries are fully charged. The mechanic inserts the hydrometer tube into the battery and withdraws a small amount of the battery acid. Small floating balls within the hydrometer tube indicate the specific gravity of the battery acid. This tells the mechanic how strong or weak the battery acid is (strong acid means a strong charge, weak acid means a weak charge).
Home hobbyists sometimes also use hydrometers. For example, people who keep salt water aquariums sometimes use a hydrometer to test how the saltiness of their aquarium water is compared with ocean water. As another example, people who brew their own beer or ferment their own wine can use a hydrometer to find out how much sugar is dissolved in the liquid. This can be useful for knowing when to stop the fermentation process, for example.
Measuring a liquid's specific gravity is also very important to some businesses. For example, it is possible to estimate the quality of crude oil by measuring its specific gravity. Lighter crude oil is good for making gasoline, butane and other light petroleum products; heavier crude oil is good for making asphalt and thick lubricating oil. The density of crude oil often determines its price. A hydrometer can be used to measure how dense (light or heavy) the crude oil is.
The basic design of laboratory hydrometers hasn't changed for a hundred years. See, for example, U.S. Pat. No. 576,537 to John Barry (1897). FIG. 1 of Mr. Barry's patent is reproduced as "prior art" FIG. 1 of this patent. The hydrometer 10 shown in Mr. Barry's patent works on the principle that a floating object displaces its weight in liquid--and will therefore float higher in liquids with greater densities. A scale 12 on Mr. Barry's hydrometer 10 indicates how high the object floats in the liquid. The scale is calibrated based on how high the object would float in pure water. One can determine the liquid's specific gravity by reading the scale to see how high the object floats in the liquid.
In more detail, prior art FIG. 1 hydrometer 10 is a sealed, graduated tube weighted at one end 14 and having a stem 16 at the other end. A graduated scale 12 is placed on or within the stem of the tube. The hydrometer 10 floats in the liquid but sinks to a depth that depends on the liquid's specific gravity. The more of stem 16 that sticks out of the liquid, the more dense the liquid. Reading the level of the liquid's surface on the stem's scale 12 indicates the liquid's specific gravity.
The prior art FIG. 1 apparatus is a "thermo-hydrometer" because it also has a thermometer 20 in the liquid. The density of a liquid can be affected by temperature. Scientists define "density" as the mass of a substance per unit volume under a specified pressure and temperature. Although changes in atmospheric pressure can generally be ignored, specific gravity is technically determined based on the density of pure water at a specific temperature (4 degrees Centigrade). To make very accurate specific gravity measurements, the scale reading must be corrected based on the temperature of the liquid being measured. Some modern hydrometers have a built-in thermometer to avoid the need for a separate thermometer.
One of the problems with the prior art FIG. 1 (or like) design is that the scale 12 on the stem 16 must be read at the air-liquid interface 22. This creates the possibility of user error. Many liquids (water, for example) tend to "wet" (stick to) the sides of both the container 24 and the sides of the hydrometer's stem 16. This means that the liquid's surface is not completely flat, but instead curves downwardly away from those surfaces to form what is called a "meniscus" (liquids that do not "wet" the container sides have an upwardly extending curved meniscus). Parallax errors and difficulties reading "through" the meniscus (especially when the liquid is not transparent) make it difficult for the user to make a correct reading. In addition to this problem, a thermo-hydrometer requires the user to read a thermometer encapsulated within the hydrometer or a separate thermometer within the liquid. This requires additional time and creates the possibility of additional user error.
In contrast, the present invention provides a differential level hydrometer that reduces the possibility for user error and provides an accurate, predictable, easy to use, reliable instrument for measuring the density (specific gravity) of a liquid.
Briefly, the differential level hydrometer provided by the present invention works by:
(a) automatically measuring the level of a liquid within a vessel; PA0 (b) floating, in the liquid, a calibrated float having a precisely known volume and density; PA0 (c) automatically measuring the level of the liquid within the vessel while the float is floating in the liquid; and PA0 (d) automatically calculating the density (specific gravity) of the liquid based on the change in levels and the volume and density of the float. PA0 The float with a calibrated volume and density can be marked with electronically readable code providing the necessary data for a density calculation. PA0 The instrument may include a device for automatically reading the code disposed on the float. PA0 A suitable device for reading electronically encoded material (e.g., bar code or magnetic media) may be placed near the top of the open vessel to recognize the coded information on the float--thus avoiding manual user entry and associated error. PA0 The vessel used to hold the liquid during measurement may be an open liquid vessel having a precise surface area perpendicular to the bottom of the vessel. PA0 The open vessel should have a precision volume, but can be of any shape so long as the surface of the area of the contained liquid is uniform and perpendicular with the bottom. PA0 A cylindrical tube shape is most likely the simplest, common volume which can be made with precise dimensions. PA0 As one example, common laboratory glassware such as a graduated cylinder could be used for the liquid vessel. PA0 A device may be coupled to the vessel to ensure the vessel is precisely aligned with the earth's gravitational pull (for example, a bubble level indicator may be used to ensure that the liquid surface is horizontal). PA0 The vessel may include a liquid interface (e.g., level) sensor which is also aligned to be perpendicular to the vessel's bottom (for example, the liquid interface sensor may be placed along one vertical side of the vessel). PA0 The instrument's liquid level interface sensor may work on optical, conductive, capacitive, sonic or other principles. PA0 A simple optical, refractive-reflective sensor capable of determining a liquid-air interface anywhere along its length, is probably the easiest and most economical device to use. However, any sensor which can accurately determine liquid level changes will work. PA0 The system may include a means for compensating for a meniscus that forms around the float. Although the liquid level sensor is exposed to the liquid and any meniscus remains constant throughout the level change, the meniscus which forms around the float must be compensated for (The height of the float's meniscus will vary with the properties of the subject liquid, and therefore cannot be assumed to be constant for different liquids). PA0 By lowering the float until the bottom contacts the liquid's surface, the volume of the float meniscus can be calculated and corrected for by the level sensor and functional logic. PA0 Other methods of meniscus correction may be made by placing gaps of known volume in the float and calculating the difference in level fluctuation as the float is lowered into the vessel. PA0 A temperature sensor disposed within the vessel may provide a temperature signal input to make appropriate temperature correction adjustments to the specific gravity reading. PA0 Since many liquids have their own temperature-density characteristics, the user can determine which temperature correction algorithm is best suited for the subject liquid. PA0 Once functional logic registers the initial liquid level, the density and volume specifications of the float, the second liquid level, meniscus correction and the temperature, the logic can calculate the density (specific gravity) and display or otherwise provide this value to the user. PA0 In the case where the user may wish to use different liquid vessels, uncoded floats, change units, choose the correct temperature algorithm, etc., a keyboard and/or computer interface may be used. PA0 The differential level hydrometer may be made as a stand alone instrument, or it may be made to interface to a computer with appropriate hardware, or it may provide both modes. PA0 In one example, the various sensors are attached to the liquid vessel in a manner which allows them to be easily removed. This makes the instrument easy to clean.
In more detail, the differential level hydrometer provided in accordance with the present invention may include a vessel having known cross-sectional characteristics. The liquid the specific gravity of which is to be measured is placed into the vessel. A gravitational reference device (e.g., a bubble level or other leveling device) may be used to ensure that the vessel is precisely aligned with reference to the earth's gravitational pull. A liquid level sensor disposed within the vessel measures the "starting" level of the liquid.
Then, a float of known dimensions and density is placed into the subject liquid to be measured. When the float has been inserted into the liquid and has come to rest, the liquid level sensor measures the level the liquid has risen to. A float of a given volume and density will float higher in (and displace less volume of) more dense liquids--causing denser liquids to rise in level less than less dense liquids.
Electronics automatically calculate and display the liquid density based on the liquid's starting level, the liquid's level with the float floating in the liquid, and the dimensions and density of the float.
The following are additional features and advantages provided in accordance with various aspects of a presently preferred example embodiment of the present invention: